1. Field of the Invention
The present invention relates to a temperature compensation method for a force sensor that measures external force through resistive elements and the force sensor.
2. Description of the Related Art
In the field of automatic operating machines, such as machine tools and robots, a force is applied to a work target or an external force is received through a work operation. In this case, it is necessary for an automatic operating machine to detect external force and moment (moment of force) applied thereto, and to perform a control in accordance with the detected force and moment. In order to perform such a control highly precisely in accordance with detected force and moment, it is necessary to precisely detect a force from the exterior (an external force) and a moment.
Hence, various kinds of force sensors are proposed so far. Conventionally known force sensors employ a fundamental configuration in which a plurality of strain detecting resistive elements are provided at deformation portions of an elastic body which elastically deforms in accordance with an external force. When an external force is applied to the elastic body of a force sensor, the plurality of strain detecting resistive elements output electrical signals in accordance with the level of deformation (a stress) of the elastic body. Based on such electrical signals, forces, etc., of equal to or greater than two components applied to the elastic body can be detected.
An example of such a force sensor is a hexaxial force sensor. Such a hexaxial force sensor divides an applied force into stress components (forces: Fx, Fy, and Fz) in individual axial directions of three axes (X axis, Y axis, and Z axis) of a Cartesian coordinate system and torque components (moments: Mx, My, and Mz) in individual axial directions, and detects those as hexaxial components.
Such a force sensor utilizes a characteristic such that the strain detecting resistive element deforms upon application of an external force and the resistance value of such element changes, and detects a change in the output voltage based on a change in the resistance value of the strain detecting resistive element, thereby measuring the magnitude of a force or a moment. The strain detecting resistive element used in the force sensor has a temperature dependency which changes the resistance value depending on an environmental temperature. Hence, when the environmental temperature changes while the force sensor is in operation, the output voltage (the output value) becomes varying even though no external force is applied, so that measurement becomes imprecise.
Accordingly, the temperature compensation circuit of the sensor disclosed in Japan Patent No. 3352006 (see claim 1) includes a temperature sensor that changes the resistance value depending on an environmental temperature, an offset correction circuit that outputs a correction value for offset correction depending on the environmental temperature detected by the temperature sensor, and a sensitivity correction circuit that outputs a correction value for sensitivity correction depending on the environmental temperature detected by the temperature sensor. By correcting the sensor output value with a correction value for offset correction and a correction value for sensitivity correction, an output value having the effect of the environmental temperature eliminated through mainly hardware processes is obtained.
However, the temperature compensation circuit of the sensor disclosed in Japan Patent No. 3352006 can eliminate the effect of the environmental temperature in a steady range where the sensor output is stable, but is unable to eliminate the effect of the environmental temperature in a transient range where the sensor output is unstable. That is, in a transient range, such as when the temperature of the temperature sensor rapidly increases right after the sensor is energized as shown in FIG. 19A, when a high-temperature object contacts the sensor and the temperature of the temperature sensor rapidly increases as shown in FIG. 19B, and when a low-temperature object contacts the sensor and the temperature of the temperature sensor rapidly decreases as shown in FIG. 19C, the output of the temperature sensor (a hardware) for detecting the environmental temperature is unstable, so that temperature compensation is difficult.